This invention generally relates to magnetic resonance imaging in a borehole.
Nuclear magnetic resonance (NMR) measurements typically are performed to investigate properties of a sample. For example, an NMR wireline or logging while drilling (LWD) downhole tool may be used to measure properties of subterranean formations. In this manner, a typical NMR tool may, for example, provide a lithology-independent measurement of the porosity of a particular formation by determining the total amount of hydrogen present in fluids of the formation. Equally important, the NMR tool may also provide measurements that indicate the dynamic properties and environment of the fluids, as these factors may be related to petrophysically important parameters. For example, the NMR measurements may provide permeability and viscosity information that is difficult or impossible to derive from other conventional logging arrangements. Thus, it is the capacity of the NMR tool to perform these measurements that makes it particularly attractive versus other types of downhole tools.
Typical NMR logging tools include a magnet that is used to polarize hydrogen nuclei (protons) in the formation and a transmitter coil, or antenna, that emits radio frequency (RF) pulses. A receiver antenna may measure the response (indicated by received spin echo signals) of the polarized hydrogen to the transmitted pulses. Quite often, the transmitter and receiver antennae are combined into a single transmitter/receiver antenna.
There are several experimental parameters that may be adjusted according to the objectives of the NMR measurement and expected properties of the formation fluids. However, the NMR techniques employed in current NMR tools typically involve some variant of a basic two step sequence that includes a polarization period followed by an acquisition sequence.
During the polarization period (often referred to as a xe2x80x9cwait timexe2x80x9d) the protons in the formation polarize in the direction of a static magnetic field (called B0) that is established by a permanent magnet (of the NMR tool). The growth of nuclear magnetization M(t) (i.e., the growth of the, polarization) is characterized by the xe2x80x9clongitudinal relaxation timexe2x80x9d (called T1) of the fluid and its maximum value (called M0), as described by the following equation:                               M          ⁡                      (            t            )                          =                              M            0                    ⁡                      (                          1              -                              ⅇ                                  -                                      t                                          T                      1                                                                                            )                                              Equation        ⁢                  xe2x80x83                ⁢        1            
The duration of the polarization period may be specified by the operator (conducting the measurement) and includes the time between the end of one acquisition sequence and the beginning of the next. For a moving tool, the effective polarization period also depends on tool dimensions and logging speed.
Referring to FIG. 1, as an example, a sample (in the volume under investigation) may initially have a longitudinal magnetization MZ 10 of approximately zero. The zero magnetization may be attributable to a preceding acquisition sequence, for example. However, the magnetization MZ 10 (under the influence of the B0 field) increases to a magnetization level (called M(tw(1)) after a polarization time tw(1) after zero magnetization. As shown, after a longer polarization time tw(2) from zero magnetization, the MZ magnetization 10 increases to an M(tw(2)) level.
An acquisition sequence begins after the polarization period. For example, an acquisition W sequence may begin at time tw(1), a time at which the magnetization MZ 10 is at the M(tw(1)) level. At this time, RF pulses are transmitted from a transmitter antenna of the tool. The pulses, in turn, produce spin echo signals 16, and the initial amplitudes of the spin echo signals 16 indicate a point on the magnetization MZ 10 curve, such as the M(tw(1)) level, for example. Therefore, by conducting several measurements that have different polarization times, points on the magnetization MZ 10 curve may be derived, and thus, the T1 time for the particular formation may be determined. A receiver antenna (that may be formed from the same coil as the transmitter antenna) receives the spin echo signals 16 and stores digital signals that indicate the spin echo signals 16.
As an example, for the acquisition sequence, a typical logging tool may emit a pulse sequence based on the CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence. The application of the CPMG pulse train includes first emitting an RF burst, called an RF pulse, that has the appropriate duration to rotate the magnetization, initially polarized along the B0 field, by 90xc2x0 into a plane perpendicular to the B0 field. The RF pulse that rotates the magnetization by 90xc2x0 is said to introduce a flip angle of 90xc2x0. Next, a train of equally spaced 180xc2x0 RF pulses is transmitted. Each 180xc2x0 RF pulse has the appropriate duration to rotate the magnet moment by 180xc2x0 to refocus the spins to generate each spin echo signal 16. Each RF pulse that rotates the magnetization by 180xc2x0 is said to introduce a flip angle of 180xc2x0. Individual hydrogen nuclei experience slightly different magnetic environments during the pulse sequence, a condition that results in an irreversible loss of magnetization and a consequent decrease in successive echo amplitudes. The rate of loss of magnetization is characterized by a xe2x80x9ctransverse relaxation timexe2x80x9d (called T2) and is depicted by the decaying envelope 12 of FIG. 1.
In general, the above NMR measurement of the T1 time may be referred to as a saturation recovery, or T1-based, measurement due to the fact that the nuclear spins are saturated (i.e., the magnetization is decreased to approximately zero) at the beginning of the wait time. Thus, from the NMR measurement, a value of the magnetization Mz 10 curve may be determined from the initial signal amplitude. In general, an NMR measurement of the signal decay may be labeled a T2-based measurement. It is noted that every T2 measurement is T1 weighted due to the fact that prepolarization occurs during the wait time before the acquisition sequence. The T2 time may be estimated from the observed decay of the envelope 12.
Referring to FIG. 2, for a particular NMR measurement, an NMR tool 30 establishes a resonance volume from which measurements of the sample are taken, such as a thin cylindrical resonance volume 32, for example. Unfortunately, the established resonance volume may be too large to yield the desired resolution. Therefore, high resolution images of the formation that surrounds the borehole may not be available. The resolution of the imaging along a longitudinal axis 34 of the borehole may be improved by decreasing the length of the RF coil. However, even with this technique, the axial resolution may be limited to approximately six to twenty-four inches. Furthermore, this technique does not provide a way to increase the resolution of the imaging in a tangential direction around the borehole.
Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are stated above.
In an embodiment of the invention, a method that is usable with a downhole NMR measurement apparatus includes transmitting RF pulses pursuant to an NMR pulse sequence into a downhole formation that surrounds the NMR measurement apparatus. In response to the RF pulses, spin echo signals are received from a region of the formation. A pulsed gradient field in the downhole formation is generated during a time period in which the RF pulses are transmitted into the downhole formation; and the generation of the gradient field is controlled to phase encode the spin echo signals for purposes of high resolution imaging of the formation.
Advantages and other features of the invention will become apparent from the following description, drawing and claims.